A typical valve assembly includes a valve body having a conical valve seat engaged with a valve secured on one end of a valve stem. The valve stem extends upwardly through a bonnet. The bonnet includes cylindrical chamber through which an intermediate portion of the valve stem extends. A valve packing is inserted into the cylindrical chamber and surrounds the valve stem to prevent fluid from leaking around the valve stem. The valve packing should be periodically replaced as part of a preventive maintenance program to assure that the valve remains in good working order.
Replacement of the valve packing is, generally speaking, a tedious and time consuming process. The valve packing usually becomes compressed and hardened when the valve has been used. Accordingly, it is often necessary to use picks, screw drivers, scrapers, wire brushes and similar instruments to remove the valve packing that are likely to scratch or damage the highly polished surfaces of the packing chamber and valve stem. Damage to those surfaces may result in the failure of the packing to provide a complete seal around the valve stem. If severe enough, damage could require replacement of the entire valve assembly.
In the past, various types of valve assemblies have been designed which include selfcontained means for removing the valve packing. For example, U.S. Pat. No. 2,809,059 discloses a valve assembly having a collar mounted in the packing chamber immediately below the valve packing. The collar includes a threaded bore which aligns with threaded bores in both the valve stem and the bonnet. In normal use, a set screw is threadably engaged in the bores formed in the housing and the collar to lock the collar in place. Thus, the collar will not move when the valve stem is rotated. When it is desired to remove the valve packing, the bore in the valve stem is aligned with the bore in the collar, and the set screw is positioned to partially engage the bores in the stem and the collar respectively. Thus, when the valve stem is rotated, the collar will move with the valve stem and lift the packing from the packing chamber. One disadvantage with this valve assembly resides in the difficulty with aligning the threaded bores in the various components. Another disadvantage is that it requires a specially designed bonnet and therefore cannot be used with existing valve assemblies which are already in place.
U.S. Pat. No. 4,394,872 discloses a valve assembly having a drive bushing mounted in the bonnet chamber below the bottom of the valve packing. A collar is integrally formed on the valve stem and is normally disposed below the drive bushing. To remove the packing, the valve stem is rotated counterclockwise to bring the collar on the valve stem into forceful engagement with the drive bushing. Continued rotation of the stem forces the packing upwardly and out of the packing chamber. This design for a valve assembly also requires a specially constructed bonnet to provide clearance for the stem collar.